A positive displacement pump makes a fluid move by trapping a fixed amount of the fluid and forcing (displacing) that trapped volume into a discharge pipe or discharge system.
Some positive displacement pumps use an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume remains constant through each cycle of pump operation.
The pump does not rely on raising the velocity of the fluid as the centrifugal pump does by moving the liquid through the impeller. Consequently, the fluid velocity inside a positive displacement pump is much lower than that of a centrifugal pump. This is often a desirable feature for certain applications, such as when needing to pump a media containing fragile solids.
Positive displacement pumps, unlike centifrugal or roto-dynamic pumps can theoretically produce the same flow at a given speed (RPM) no matter what the discharge pressure. Therefore, positive displacement pumps can be regarded as constant flow devices. However, a slight increase in internal leakage as the pressure increases can prevent a truly constant flow rate.
In application, a positive displacement pump must not be allowed to operate against a closed valve on the discharge side of the pump, because it has no shutoff head like a centrifugal pump.
With the pump operating against a closed discharge valve, it will continue to produce flow and the pressure in the discharge line will increase until the pipeline fractures is severely damaged. To prevent this, a relief or safety valve on the discharge side of the positive displacement pump is therefore often necessary. This relief valve can be positioned either internally or externally to the pump.